Sensing operations in a memory device

ABSTRACT

Methods for sensing, method for programming, memory devices, and memory systems are disclosed. In one such method for sensing, a counting circuit generates a count output and a translated count output. The count output is converted into a time varying voltage that biases a word line coupled to memory cells being sensed. Target data for each memory cell is stored in a data cache associated with that particular memory cell. When it is detected that a memory cell has turned on, the translated count output associated with the count output that is indicative of the voltage level that turned on the memory cell is compared to the target data. The comparison determines the state of the memory cell.

RELATED APPLICATION

This is a Divisional of U.S. application Ser. No. 12/720,239, titled“SENSING OPERATIONS IN A MEMORY DEVICE,” filed Mar. 9, 2010 and issuedas U.S. Pat. No. 8,242,523 on Aug. 14, 2012, which is commonly assignedand incorporated herein by reference.

TECHNICAL FIELD

The present embodiments relate generally to memory and a particularembodiment relates to sensing operations in a memory.

BACKGROUND

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include personal computers, flash drives,digital cameras, and cellular telephones. Program code and system datasuch as a basic input/output system (BIOS) are typically stored in flashmemory devices for use in personal computer systems.

A flash memory is a type of memory that can be erased and reprogrammedin blocks instead of one byte at a time. A typical flash memorycomprises a memory array, which includes a large number of memory cells.Changes in threshold voltage of the memory cells, through programming ofcharge storage nodes (e.g., floating gates or charge traps) or otherphysical phenomena (e.g., phase change or polarization), determine thedata value of each cell. The cells are usually grouped into blocks. Eachof the cells within a block can be electrically programmed, such as bycharging the charge storage node. The data in a cell of this type isdetermined by the presence or absence of the charge in the chargestorage node. The charge can be removed from the charge storage node byan erase operation.

Each memory cell can be programmed as a single bit per cell (i.e.,single level cell—SLC) or multiple bits per cell (i.e., multilevelcell—MLC). Each cell's threshold voltage (V_(t)) is representative ofthe data that is stored in the cell. For example, in a single bit percell, a V_(t) of 1.5V can indicate a programmed cell while a V_(t) of−0.5V might indicate an erased cell.

A multilevel cell has multiple V_(t) ranges that each indicates adifferent state. Multilevel cells can take advantage of the analognature of a traditional flash cell by assigning a bit pattern to aspecific V_(t) range for the cell. This technology permits the storageof data values representing two or more bits per cell, depending on thequantity of V_(t) ranges assigned to the cell.

As the performance of processors increases, the performance of thememory coupled to the processor should also increase, without impactingprogram or read reliability, to keep from becoming a bottleneck duringdata transfers. The density of flash memory arrays has also historicallybeen increasing by increasing the quantity of bits storable in eachmemory cell. This results in greater quantities of data to betransferred to the memory array and programmed within a certain timeperiod.

For the reasons stated above, and for other reasons stated below thatwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternative methods and apparatus for sensing memory cells in a memorydevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of one embodiment of a portion of amemory array.

FIG. 2 shows a block diagram of one embodiment of odd and even sensingpaths in a memory device.

FIG. 3 shows a block diagram of one embodiment of a data cache inaccordance with the sensing path of FIG. 2.

FIG. 4 shows a flow chart of one embodiment of a program operation.

FIG. 5 shows a flow chart of one embodiment of a program verifyoperation in accordance with the program operation of FIG. 4.

FIG. 6 shows a flow chart of one embodiment of a read operation.

FIG. 7 shows a combination timing diagram and threshold voltage rangedistribution of one embodiment for sensing operations in a memorydevice.

FIG. 8 shows a combination timing diagram and threshold voltage rangedistribution of an alternate embodiment for sensing operations in amemory device.

FIG. 9 shows a block diagram of one embodiment of a memory system.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof.

FIG. 1 illustrates a schematic diagram of a portion of a NANDarchitecture memory array 101 comprising series strings of non-volatilememory cells that can be sensed using the embodiments of the sensingoperation discussed subsequently. While the subsequent discussions referto a NAND memory device, the present embodiments are not limited to suchan architecture but can be used in other memory device architectures aswell.

The array can be comprised of an array of non-volatile memory cells 101(e.g., floating gate) arranged in columns such as series strings 104,105. Each of the cells 101 are coupled drain to source in each seriesstring 104, 105. An access line (e.g. word line) WL0-WL31 that spansacross multiple series strings 104, 105 is coupled to the control gatesof each memory cell in a row in order to bias the control gates of thememory cells in the row. Data lines, such as bit lines BL1, BL2 arecoupled to the strings and eventually coupled to sense amplifiercircuitry, as shown in FIGS. 2 and 3, that detect the state of each cellby sensing current or voltage on a particular bit line.

Each series string 104, 105 of memory cells is coupled to a source line106 by a source select gate 116, 117 and to an individual bit line BL1,BL2 by a drain select gate 112, 113. The source select gates 116, 117are controlled by a source select gate control line SG(S) 118 coupled totheir control gates. The drain select gates 112, 113 are controlled by adrain select gate control line SG(D) 114.

Even though the subsequently described sensing embodiments refer tosingle level cell (SLC) embodiments, each memory cell can be programmedas an SLC or multiple level cell (MLC). Each cell's threshold voltage(V_(t)) is indicative of the data that is stored in the cell. Forexample, in an SLC, a V_(t) of 1.5V might indicate a programmed cellwhile a V_(t) of −0.5V might indicate an erased cell. The MLC usesmultiple V_(t) ranges that each indicate a different state. Multilevelcells can take advantage of the analog nature of a traditional flashcell by assigning a bit pattern to a specific V_(t) range. Thistechnology permits the storage of data values representing two or morebits per cell, depending on the quantity of V_(t) ranges assigned to thecell.

The embodiments of the sensing operations described subsequently use atime varying access line voltage (e.g., ramped word line voltage)architecture to turn on select memory cells during the sense operation.The ramped voltage, generated by a counter and a digital-to-analogconverter, for example, can be applied to any type of sensing operation.For example, both read operations and program verify operations areencompassed by these embodiments.

FIG. 2 illustrates a block diagram of one embodiment of odd 201 and even202 sensing paths of a memory device. The odd sensing path 201 is usedfor sensing odd column addresses while the even sensing path 202 is usedfor sensing even column addresses.

Each sensing path 201, 202 includes, for the embodiment depicted in FIG.2 having eight data words, eight data cache circuits 219, 220 and 221,222 a multiplexing circuit 230, 231 that, in one embodiment, selectsbetween 32 bit lines. Each sensing path 201, 202 of FIG. 2, for purposesof clarity, only shows two data cache circuits 219, 220 and 221, 222,respectively, for each sensing path. Each multiplexing circuit 230, 231can include, for such an embodiment, eight 4:1 multiplexers that areeach coupled to four bit line inputs and one data cache circuit on theoutput so that one bit line is selected from the four input bit linesfor each multiplexer. Each row of data cache circuits is coupled to adata output DQ0-DQ7 of the memory device. An example of one data cache219 and multiplexing circuit 230 is illustrated in FIG. 3 and discussedsubsequently.

The sensing paths 201, 202 are also coupled to peripheral circuitry thatincludes a counting circuit 200 (e.g., 8-bit counter and logic block), adigital-to-analog converter (DAC) circuit 210, and a high voltagedecoder circuit 215. The counting circuit 200 is clocked by a clockinput CLK (e.g., 160 ns period). One bit of the 8-bit count (e.g.,COUNTER<0>) is used as a clock (e.g., 320 ns period) for clockingcircuitry in the data cache circuits 219-222.

The seven remaining bits of the 8-bit count (e.g., COUNTER<7:1>) areoutput to the DAC 210 that acts as a voltage generation circuit. In oneembodiment, the counting circuit 200 counts from 0 to 127. The DAC 210generates a time varying signal by converting the digital count, overtime, to an analog voltage signal such as a ramped voltage (e.g., a0V-5V ramped voltage).

The ramped voltage is input to the high voltage decoder circuit 215 thatapplies the ramped voltage to a selected word line. The high voltagedecoder circuit 215 has pre-decoded address inputs that enable the highvoltage decoder circuit 215 to determine which block and which word linein the block is to be biased with the ramped voltage.

The output of the high voltage decoder circuit 215 is input to a stringdriver circuit 203. The string driver circuit 203 drives the word linesand passes different levels of voltages during program, read, andprogram verify operations. In one embodiment, one out of 64 word linesin a block is biased with the ramped voltage during sensing while theunselected word lines in a block are biased with a pass voltage V_(pass)selected to activate memory cells coupled to those unselected word linesregardless of their data values, e.g., a V_(pass) voltage from 5V to 6V(depending on the embodiment).

The logic block portion of the counting circuit 200 is responsible forgenerating a translated count (e.g., CNTR<6:0> data) from theCOUNTER<7:1> count of the 8-bit counter. The logic block portion cantranslate the COUNTER<7:1> output as follows:

-   CNTR<0>=COUNTER<1>; CNTR<1>=COUNTER<2>;-   CNTR<2>=COUNTER<3>;-   CNTR<3>=COUNTER<4>; CNTR<4>=COUNTER<5>; and CNTR<5>=-   COUNTER<6>. CNTR<6> can be set according to the sense operation    being performed and/or the count value of COUNTER<7:1>, for example.

For example, if a program verify operation is being performed, CNTR<6>can be set to a logic 1. During a read operation, CNTR<6> can be set toa logic 0 if COUNTER<7:1> is less than a threshold count value (e.g.,16) and CNTR<6> can be set to a logic 1 if COUNTER<7:1> is greater thanor equal to the threshold count value (e.g. 16). The values for CNTR<6>are subsequently discussed in more detail.

The peripheral circuitry is further comprised of additional multiplexingcircuitry comprising a plurality of multiplexers 250-255 that are eachconfigured to select between an input data signal from the DQ0-DQ7 datainputs of the memory device and a respective counter bit (e.g., CNTR<x>)from the 8-bit counter circuit 200. The output of each multiplexer250-255 of the multiplexing circuitry is input to a different latchLAT0-LAT6 of each data cache circuit 219-222.

For example, referring to the DQ<0> data cache circuits 219, 221,CNTR<6:1> and DQ<0> are input to the multiplexers 250-252 so thatcontrol circuitry (not shown in FIG. 2) can select between latchinginput data from DQ<0> or a counter output bit to its respective latchLAT0-LAT6. Similarly, referring to the DQ<7> data cache circuits 220,222, CNTR<6:1> and DQ<7> are input to the multiplexers 253-255 so thatthe control circuitry can select between latching input data from DQ<7>or a counter output bit to its respective latch LAT0-LAT6. As will bediscussed subsequently, the latched data from the DQ inputs are targetcount data that are compared to the counter output CNTR during a senseoperation in order to determine a present voltage of the ramped wordline voltage and, thus, a state of a sensed memory cell.

FIG. 3 illustrates a block diagram of one embodiment of a data cachecircuit 219 that can be incorporated in the sensing paths of theembodiment of FIG. 2. FIG. 3 also illustrates a multiplexing circuit 230as discussed previously with reference to FIG. 2. This circuit 230 canbe configured for bit line multiplexing as well as controlling bit linebiasing during sensing or programming operations.

The data cache 300 is comprised of a sense circuit (e.g., senseamplifier circuitry) 301 coupled to a bit line control circuit 303 thatis coupled to a pulse generator 305 that is coupled to a data latchcircuit having data latches and a comparator 307. The data cache 300 isrepeated multiple times (e.g., 8 times) in the y-direction. A columnselect circuit 309 is coupled to the last data cache 300 in the column.

The sense amplifier circuitry 301 detects either current or voltage on abit line selected by the multiplexing circuit 230. The detected currentor voltage is an indication whether the selected memory cell has beenturned on by the ramped voltage signal applied to its control gate.

The bit line control circuit 303 includes a pass/fail latch for programverify and erase verify operations. When one of these verify operationshas passed, the latch is set to indicate a successful verify.

The pulse generator circuit 305 generates a synchronous pulse wheneverthe sense amplifier circuitry 301 detects a current/voltage. In otherwords, when a selected memory cell turns on from a particular voltagebiasing the control gate of the selected memory cell, a current flows inthe bit line. This current is detected by the sense amplifier circuitry301 that causes the pulse generator to generate a synchronous pulseindicating that the memory cell has turned on.

The data latch circuit 307 of the data cache 300 can be comprised of aplurality of data latches (e.g., seven latches) and a comparatorcircuit. The data latches can store a digital representation of a targetthreshold voltage that is loaded from one of the data inputs DQ0-DQ7 ofthe memory device (e.g. a target count). The digitally representedtarget threshold voltage is the threshold voltage to which a memory cellis to be programmed.

The column select circuit 309 is coupled to the output of the data latchcircuit. During a sense operation, the column select circuit 309 selectsgroups of columns based on an input address and an indication from thedata latch circuit 307 comparator that a selected memory cell has beenprogrammed to its target threshold voltage.

FIG. 4 illustrates a flow chart of one embodiment of a method forprogramming a memory device. This method also refers to the circuits ofFIGS. 2 and 3 to describe the execution of the program operation.

An initial program command is transmitted to the memory device thatreceives and decodes the command 401. Data to be programmed is thentransmitted to the memory device 403 for programming. In one embodiment,2 k bytes of data are loaded sequentially and programmed simultaneously.

It is determined whether a logical 0 or a logical 1 is being programmedto each memory cell 405. If a logical 1 is being programmed to thememory cell, the logical 1 is loaded into the most significant bit(LAT<6>) of the latches 307 as illustrated in FIG. 3. At the same time,the other latches (LAT<5:0> for that memory cell are also loaded withthe data that will be used during a program verify operation. In thecase of a programmed logical 1, the latches LAT<6:0> are loaded 409 withprogram verify data representative of a desired threshold voltage. Forexample, the latches LAT<6:0> could be loaded 409 with a logical 1010000as the program verify data. The program verify data is representative ofone embodiment of the lowest threshold voltage that will be verified asa programmed logical 1.

The logical 1 loaded to the LAT<4> location causes the programmed memorycell to be verified to a higher threshold voltage which provides amargin between a read operation and a program verify operation. The1010000 program verify data written to the latches can be altered bydata stored in registers typically referred to as trim data. The trimdata can increase or decrease the voltage to which a memory cell isprogram verified by altering the data stored in the latches LAT<6:0>.Thus, by altering the program verify data prior to being stored in thelatches, the threshold voltage range for the programmed state can beadjusted in both voltage level and resolution.

If a logical 0 was written to the memory cell, the logical 0 is loadedinto the most significant bit (LAT<6>) of the latches 307 as illustratedin FIG. 3. In an embodiment where the erased cell does not need to beprogrammed (e.g., to a less negative threshold), none of the otherlatches need to be set to a logical 1. Thus, the latches LAT<6:0> for aprogrammed 0 state can be loaded 407 with a logical 0000000 as theprogram verify data.

The programming of the memory cells is then performed by the programpulses 411 applied to the memory cell's control gate through theselected word line. These pulses might start with an amplitude of 15Vand incrementally increase to 20V in order to increase the thresholdvoltage of a memory cell that is being programmed with a logical 1. Aprogram verify operation 413 is performed after each program pulse. Oneembodiment of a program verify operation is illustrated by the flowchart of FIG. 5 and described subsequently.

If the program verify operation passes 415, the program operation issuccessful and has been completed. If the program verify operationfails, the previous programming pulse voltage is incremented by a stepvoltage and that higher voltage programming pulse is applied to thememory cell 411. The program pulse/verify operation is repeated untilthe memory cell passes the program verify operation or the memory cellis flagged as not programmable.

FIG. 5 illustrates a flow chart of one embodiment of a program verifyoperation in accordance with the program operation of FIG. 4. Referringto both FIG. 5 and the block diagram of FIG. 2, it should be noted thatthe COUNTER output coupled to the digital-to-analog converter is not thesame as the CNTR output coupled to the latches of the data cachecircuitry. One such translation from COUNTER<7:1>to CNTR<6:0>was shownand discussed previously with reference to FIG. 2.

The counting circuit starts 501. As discussed previously, COUNTER<0>clocks the data cache circuits. COUNTER<7:1> increments from 0 to 127and is coupled to the digital-to-analog converter to generate the rampedvoltage. By forcing CNTR<6> to a logical 1, the count CNTR<6:0> sent tothe latches for comparison is actually counting from 64 to 127 in thisexample.

The ramped voltage from the digital-to-analog converter that is appliedto the selected word line starts to ramp from a voltage below the firstprogrammed level (e.g., 0V) to a maximum voltage (e.g., 5V), based onthe COUNTER<7:1> value 503. When the word line voltage reaches thethreshold voltage to which a selected memory cell has been programmed,it turns on. The memory cell turning on causes a bit line voltage or acurrent to flow on the bit line that is detected by the sense amplifiercircuitry 505.

A comparison is then performed 507 between CNTR<6:0>and a target countvalue stored in LAT<6:0>. The target count value can be program verifydata. If CNTR<6:0>is greater than or equal to LAT<6:0>, the memory cellhas passed the program verify operation 509. If the CNTR<6:0>is lessthan LAT<6:0>, the selected memory cell fails the program verifyoperation 511 and should be biased with at least one additionalprogramming pulse as discussed in the programmed operation of FIG. 4.

In one embodiment, memory cells that have LAT<6:0> less than 1010000(i.e., 80 in decimal) need additional programming until their respectivesense amplifier circuitry detects a current at a CNTR value greater thanor equal to 80. Since erased memory cells are at a logical 0 state andhave LAT<6:0>=0000000, in at least some embodiments, these memory cellsalways pass the verify operation.

FIG. 6 illustrates a flow chart of one embodiment of a read operation.After the memory device has received and decoded a read command 601, thecounter COUNTER<7:0> starts counting 603. In one embodiment,COUNTER<7:1> goes from 0 to 127. Alternate embodiments might use othercount values.

The voltage from the digital-to-analog converter is applied to theselected word line 605 such that a ramped voltage biases the controlgate of the memory cell selected to be read. When the ramped voltagereaches the threshold voltage of the selected memory cell, the memorycell turns on thus causing a current to flow on the bit line. Therespective sense amplifier circuitry detects the memory cell turning on607 (e.g., current or voltage) that causes a comparison to occur betweenthe CNTR<6:0> and the LAT<6:0> 609 in which a threshold count value hasbeen stored.

If the CNTR<6:0> is less than the threshold count value (e.g., 16), alogical 0 has been stored in the data cache latch LAT<6> and the memorycell can be read as being in a logical 0 state 611. If the CNTR<6:0> isgreater than or equal to the threshold, a logical 1 has been stored inthe data cache latch LAT<6> and the memory cell can be read as being alogical 1 state 613. In another embodiment, the counter value could beused as the data value.

FIG. 7 illustrates a combination timing diagram and threshold voltagerange distribution of one embodiment of a sensing operation in a memorydevice, such as the non-volatile memory device of FIG. 1. The thresholdvoltage ranges 700, 701 of the first waveform are shown in relation to0V. In the illustrated embodiment, an erased memory cell is read as alogical 0. For example, a cell having a threshold voltage within anegative threshold voltage range 700. The programmed state of the memorycell is read as a logical 1. For example, a cell having a thresholdvoltage within a positive threshold voltage range 701. The V_(t) ranges700, 701 are plotted with the threshold voltage along the x-axis and thenumber of cells of each V_(t) along the y-axis. The SLC states used inthe embodiments illustrated in FIGS. 7 and 8 are the opposite of atypical prior art SLC memory cell in which the erased state is read as alogical 1.

By setting program verify data CNTR<6:0> to 1010000 (i.e., 80 decimal),as described previously with reference to FIG. 5, a margin 720 has beencreated between 0V and the lowest threshold voltage of the programmedthreshold voltage range 701. For example, if CNTR count 0000000corresponds to 0V then 1010000 might correspond to 0.6V thus creating a0.6V margin between the lowest programmed V_(t) and the read voltage(assuming the read voltage is 0V), thus mitigating any shifts in datavalues after programming.

The solid line in the second waveform shows that the most significantbit of the translated counter output CNTR<6> coupled to the latches canbe set high 703 after, for example, count 16 of COUNTER<7:1> for a readoperation as described previously. As shown by the dashed line, in atleast one embodiment, this bit CNTR<6> can always be high 702 during aprogram verify operation.

The third waveform shows the selected word line ramp voltage as itincreases as the CNTR<6:0> to the latches counts from 0 to 127.Alternate embodiments could count to different maximum values instead of127 for embodiments having a ramped voltage, or other time varyingvoltage, that goes to a different maximum voltage (e.g., 2V-3V).

FIG. 8 illustrates a combination timing diagram and threshold voltagerange distribution of an alternate embodiment of a sensing operation ina memory device, such as the non-volatile memory device of FIG. 1. Thisembodiment differs from the embodiment illustrated in FIG. 2 in that theramped word line voltage starts as a negative value (e.g., −3V) andramps to a positive value (e.g., 5V).

The first waveform shows that the erased state is a logical 0 but has amore negative threshold voltage range 800 than the previous embodiment.In this embodiment, the lowest threshold of the erased threshold voltagerange is around −3V. Thus, the ramped voltage should start at the morenegative voltage. The programmed state is a logical 1 and has a positivethreshold voltage range 801. The first waveform also shows that byforcing the program verify data to 1001000 instead of 1000000, a marginis created between 0V and the lowest threshold voltage of the programmedthreshold range 801.

The next line shows the digital data associated with the COUNTER<7:1>output of the counting circuit. In this embodiment, the logic 0 of theerased state is set into bit COUNTER<7> of the erased digital data. Thelogic 1 of the programmed state is set into bit COUNTER<7> of theprogrammed digital data.

The third waveform of FIG. 8 shows the selected word line rampedvoltage. In one embodiment, this ramped voltage goes from −3V to 5V.Alternate embodiments can use other start and stop voltages.

The final waveform shows the CNTR<6:0> output from the logic blockportion of the counting circuit of FIG. 2. This waveform shows thatCNTR<6:0> goes from 0-63 for the erased state and 64-127 for theprogrammed state.

FIG. 9 illustrates a functional block diagram of a memory device 900.The memory device 900 is coupled to an external processor 910. Theprocessor 910 may be a microprocessor or some other type of controller.The memory device 900 and the processor 910 form part of a memory system920. The memory device 900 has been simplified to focus on features ofthe memory that are helpful in understanding the present invention.

The memory device 900 includes an array 930 of non-volatile memorycells, such as the one illustrated previously in FIG. 1. The memoryarray 930 is arranged in banks of word line rows and bit line columns.In one embodiment, the columns of the memory array 930 are comprised ofseries strings of memory cells. As is well known in the art, theconnections of the cells to the bit lines determines whether the arrayis a NAND architecture, an AND architecture, or a NOR architecture.

Address buffer circuitry 940 is provided to latch address signalsprovided through I/O circuitry 960. Address signals are received anddecoded by a row decoder 944 and a column decoder 946 to access thememory array 930. It will be appreciated by those skilled in the art,with the benefit of the present description, that the number of addressinput connections depends on the density and architecture of the memoryarray 930. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 900 reads data in the memory array 930 by sensingvoltage or current changes in the memory array columns using senseamplifier circuitry 950. The sense amplifier circuitry 950, in oneembodiment, is coupled to read and latch a row of data from the memoryarray 930. Data input and output buffer circuitry 960 is included forbidirectional data communication as well as the address communicationover a plurality of data connections 962 with the controller 910. Writecircuitry 955 is provided to write data to the memory array.

Memory control circuitry 970 decodes signals provided on controlconnections 972 from the processor 910. These signals are used tocontrol the operations on the memory array 930, including data read,data write (program), and erase operations. The memory control circuitry970 may be a state machine, a sequencer, or some other type ofcontroller to generate the memory control signals. In one embodiment,the memory control circuitry 970 is configured to execute the senseoperations of the memory device as described previously.

The flash memory device illustrated in FIG. 9 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of flashmemories are known to those skilled in the art.

CONCLUSION

In summary, one or more embodiments of the sensing operation can providereduced time for verify and read operations while, in at least oneembodiment, also producing only positive voltages for the sensingoperation. In the case of a verify operation, for example, this can beaccomplished by writing digital data into a series of latches where thedigital data is indicative of the data stored in a corresponding memorycell. A counter and digital-to-analog converter can be used to generatea time varying voltage that is applied to a selected word line coupledto the corresponding memory cell. A count value associated with thecounter can be compared to the digital data during a program verifyoperation to determine if the program operation was successful.Similarly, during a read operation, if the count value that generatesthe voltage that turns on the corresponding memory cell is greater thana threshold, the selected memory cell is considered programmed.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

What is claimed is:
 1. A method for sensing a memory cell in a memorydevice, the method comprising: generating an increasing count;generating a ramped voltage signal, wherein a voltage of the rampedsignal increases with each increased count; biasing the memory cell withthe ramped voltage; determining a first count associated with the rampedvoltage signal responsive to turning on the memory cell; comparing thefirst count to a second count associated with the memory cell; anddetermining a state of the memory cell in response to the comparison. 2.The method of claim 1 and further including loading the second countinto latches associated with the memory cell.
 3. The method of claim 2and further comprising altering the second count prior to loading intothe latches.
 4. The method of claim 2 wherein the second count comprisesa target count, and wherein the target count is loaded during aprogramming operation.
 5. The method of claim 1 wherein each count is adigital count and generating the ramped voltage comprises executing adigital-to-analog conversion process on the increasing count, whereinthe first count is translated from the increasing count.
 6. The methodof claim 1 wherein the second count includes a most significant bit thatis set to a same value as data to be programmed into the memory cell. 7.The method of claim 1 wherein the ramped voltage signal comprises onlypositive voltages.
 8. The method of claim 1 and further comprising:translating the increasing count to the first count; setting a mostsignificant bit of the translated count to a first data value when themethod for sensing is a program verify operation; setting the mostsignificant bit of the translated count to a second data value if theincreasing count is less than a threshold and the method for sensing isa read operation; and setting the most significant bit of the translatedcount to the first data value if the increasing count is greater than orequal to the threshold and the method for sensing is a read operation.9. A memory device comprising: a memory array comprising a plurality ofmemory cells; a counting circuit configured to generate a count outputand a translated count output; a digital-to-analog converter coupled tothe counting circuit and configured to generate a ramped voltage inresponse to the count output, the ramped voltage being selectivelycouplable to control gates of the memory cells; a plurality of datacaches wherein each data cache is configured to store a count and,during a sense operation, detect when a selected memory cell coupled tothe respective data cache turns on in response to a particular voltageof the ramped voltage and compare the count with the translated countoutput, associated with the count output responsible for the particularvoltage, to determine a state of the memory cell; and memory controlcircuitry coupled to the memory array and configured to control thesense operation of the memory array.
 10. The memory device of claim 9wherein the memory control circuitry is further configured to control aprogram operation that stores the count in each data cache such that amost significant bit of the count stored in a first data cache matchesdata to be programmed in the memory cell associated with the respectivedata cache.
 11. The memory device of claim 9 wherein one bit of thecount output is used as a clock (e.g., 320 ns period) for clockingcircuitry in the plurality of data caches.
 12. The memory device ofclaim 9 and further comprising a data cache having a sense circuitconfigured to detect whether a selected memory cell has been turned onby the ramped voltage.
 13. The memory device of claim 9 and furthercomprising a data latch circuit, wherein the data latch circuitcomprises a plurality of data latches and a comparator.
 14. The memorydevice of claim 9 wherein the ramped voltage starts as a negativevoltage and ramps to a positive voltage over time.
 15. The memory deviceof claim 9 wherein the ramped voltage starts at 0V and ramps to apositive voltage over time.
 16. A method for sensing a memory cell in amemory device, the method comprising: generating a ramped voltageincreasing in response to an increasing count of a first counter;biasing the memory cell with the ramped voltage; and determining a stateof the memory cell in response to a comparison between a translation ofthe count of the first counter and a target count.
 17. The method ofclaim 16, wherein the target count is saved in latches associated withthe memory cell.
 18. The method of claim 17, wherein the target count issaved in the latches during a programming operation.
 19. The method ofclaim 16, wherein the count of the first counter and the target countare digital counts, and wherein generating the ramped voltage comprisesexecuting a digital-to-analog conversion process on the increasing countof the first counter.
 20. A method for sensing a memory cell in a memorydevice, the method comprising: generating a ramped voltage increasing inresponse to an increasing count of a first counter; biasing the memorycell with the ramped voltage; and determining a state of the memory cellin response to a comparison between the count of the first counter and atarget count; wherein the count of the first counter and the targetcount are digital counts, and wherein generating the ramped voltagecomprises executing a digital-to-analog conversion process on theincreasing count of the first counter, wherein the first count istranslated from the increasing count; and wherein the target countincludes a most significant bit set to a same value as data to beprogrammed into the memory cell, the method further comprising:translating the count of the first counter to a translated first count;setting a most significant bit of the translated first count to a firstdata value when the method for sensing is a program verify operation;setting the most significant bit of the translated first count to asecond data value when the count of the first counter is less than athreshold and the method for sensing is a read operation; and settingthe most significant bit of the translated count to the first data valuewhen the count of the first counter is greater than or equal to thethreshold and the method for sensing is a read operation.